Manufacturing of semiconductor devices employs a thin gate dielectric (typically silicon dioxide) between the underlying silicon substrate and the gate electrode. Depositing a thin dielectric film on a silicon substrate forms a gate dielectric. Typical processes for growth of dielectric films include oxidation, chemical vapor deposition and atomic layer deposition processes. As integrated circuit devices shrink, the thickness of the gate dielectric needs to shrink proportionally. However, semiconductor manufacturers have reached the limit to which the thickness of conventional gate dielectric materials can be decreased without compromising the electrical characteristics. Rather than degrading the dielectric properties by using a silicon dioxide dielectric that is only a few atomic layers thick, equivalent dielectric performance can be achieved by substituting the silicon dioxide for a thicker layer of a new material exhibiting a higher dielectric constant. Therefore, new compositions or methods to produce a dielectric film with a higher dielectric constant than silicon dioxide (referred to as “high-k dielectrics”) are required. These “high-k dielectrics” must have a low leakage current through the gate dielectric. Thus, it is desirable to develop new compositions and methods of depositing dielectric films with the required higher dielectric properties so that films with more than one or two layers of atoms can be deposited. Due to the requirements for thin dielectric films, having uniform coverage of material that is very high quality is critical to the performance of the gate dielectric.
Of particular interest is the formation of metal silicon oxynitride (“MSiON”) films. Forming a MSiON dielectric film typically involves feeding a metal source, a silicon source, an oxygen source and a nitrogen source (collectively referred to herein as the “dielectric precursors”) in the proper relative amounts to a deposition device wherein a silicon substrate is held at an elevated temperature. The dielectric precursors are fed to a deposition chamber through a “delivery system.” A “delivery system” is the system of measuring and controlling the amounts of the various dielectric precursors being fed to the deposition chamber. Various delivery systems are known to one skilled in the art. Once in the deposition chamber, the dielectric precursors react to deposit a dielectric film on the silicon substrate in a “forming” step. A “forming” step or steps, as used in this application, is the step or steps wherein materials are deposited on the silicon substrate or wherein the molecular composition or structure of the film on the silicon substrate is modified. The “desired final composition” of the dielectric film is the precise chemical composition and atomic structure of the gate dielectric after the last forming step is complete. Compounds of hafnium, such as hafnium oxides, hafnium silicates and hafnium silicon oxy nitrides are currently the most promising high-k gate dielectric choices. The metal source for the forming process is typically a liquid precursor solution containing the desired metal in a solvent. Similarly, the silicon sources available in the art prior to the current invention typically use a liquid precursor containing the desired silicon compound in a solvent.
U.S. Patent Publication No. U.S. 2003/0207549, PAJ Patent Application No. 2000272283, U.S. Pat. No. 6,399,208, and U.S. Patent Publication No. 2003/0207549 disclose information relevant to forming dielectric films. However, these references suffer from one or more of the disadvantages discussed below.
Some gate dielectric-forming processes require multiple forming steps. For instance, a dielectric film may be formed by depositing a metal and silicon on a substrate in a first step followed by a second “post deposition step” wherein the composition or structure of the deposited metal/silicon film is modified to achieve the desired final composition of a MSiON gate dielectric film. An example of a post deposition step is rapid thermal annealing in an environment that is filled of nitrogen or ammonia. Because control of the film composition is important in dielectric film deposition processes, the fewer the steps, the better the control of the process, and the higher the quality (reflected by dielectric constant, density, contamination, composition and other quality control properties) and conformality (the ability of the film to evenly deposit on all surfaces and shapes of substrate) of the dielectric film.
It is known in the art that any silicon sources that contain carbon in the ligands can lead to carbon in the film and result in degraded electrical properties. Furthermore, any chlorine incorporated in dielectric films is undesirable due to its harmful effect on the electrical properties of the film and the stability of the chlorine in the film (the stability makes it hard to remove chlorine from the dielectric film). Also, the presence of chlorine in the silicon or metal source results in the generation of chloride based particulates in the reaction chamber and deposits in the exhaust system. Thus, to achieve the ideal electrical properties and to minimize particulate generation and tool downtime due to exhaust system cleaning, it is desirable to deposit dielectric films from precursors free of carbon or chlorine in the atomic structure.
The physical properties of a MSiON dielectric film are affected by the metal (M) to silicon (Si) ratio, or M/Si. It is desirable to be able to control the M/Si ratio over a broad range. Thus, it is important to be able to vary the metal and silicon feed independently to achieve the broadest possible M/Si ratios. Some processes use a silicon source precursor that also contains some amount of the metal that is to be deposited. The problem encountered is that changes in the metal-containing silicon source precursor feed rate changes the total amount of the metal fed to the process (due to the metal contained in the silicon precursor). This limits the controllability of the deposition process because the silicon feed rate cannot be changed without also affecting the total amount of metal being fed to the deposition chamber. Furthermore, the ratio of M/Si that can be fed is limited by the composition of the metal in the silicon source precursor. Thus a change in desired M/Si ratio can require changing precursor solutions being fed to the process.
Vaporizing silicon precursor streams can also lead to problems with film composition control. Referring to FIG. 2, some processes in the art use a vaporizer to vaporize the liquid silicon source. The vaporized stream is then fed to the deposition chamber. When the metal source and the silicon source are supplied in liquid form, they must both be vaporized before being introduced into the deposition chamber. Vaporizing two different streams can lead to variable feed concentrations and formation of silicon or metal residues in the vaporizer that can adversely affect the conformality of the film composition. Differences in vaporization of the silicon and metal sources can also lead to compositional gradients in the dielectric.
Bubbling a carrier gas through a liquid precursor can also cause quality problems. In some processes, a silicon source is fed by bubbling a carrier gas through a liquid silicon source. A vaporizer is not used in these processes because the vapor pressure of the silicon source is high enough to be transported as a vapor in a mixture with the carrier gas. In these processes, the composition of the stream transporting the silicon source to the deposition chamber can vary with temperature and pressure in the bubbling system. This variability in stream composition leads to variability in the composition of the dielectric film, which is a significant quality control issue.
For the foregoing reasons, it is desirable to form a dielectric film of the final desired composition in a single forming step. Furthermore, the film should be free of any chlorine and contain as little carbon as possible in the molecular structure. It is also desirable to use a silicon source that is free of any deposition metals so the silicon source feed and the metal source feed may be independently controlled. Finally, it is desirable to have a silicon source that is in the vapor phase at process feed conditions to avoid the need to vaporize a liquid silicon source or bubble a carrier gas through a liquid source.